Head clamp for imaging and neurosurgery

ABSTRACT

A head clamp for neurosurgery is described that includes a member for at least partially encircling the head of a subject. The first end and second end of the head clamp include first and second skull attachment portions for attaching the member to the head of a subject. A position setter, such as an indexing mechanism, is also provided that allows the member to be moved or indexed between at least two repeatable relative positions.

This is a Continuation of U.S. application Ser. No. 13/320,953 filedNov. 17, 2011, which in turn is a National Phase of InternationalApplication No. PCT/GB2010/001003 filed May 20, 2010, which claims thebenefit of British Application No. 0908787.5 filed on May 21, 2009. Thedisclosures of the prior applications are hereby incorporated byreference herein in their entireties.

The present invention relates to a clamp for attachment to the head of asubject, and in particular to an indexable head clamp suitable for usein both magnetic resonance imaging (MRI) of the head and stereotacticneurosurgery procedures.

It is known to provide various types of head clamp to grip and stabilisethe head of a subject during neurosurgical procedures. One known type ofhead clamp comprises a generally c-shaped frame member for partiallyencircling the head of a subject with a pin or pins provided at each endof the frame member to engage the skull. Such head clamps are placed inthe desired position relative to the head before being firmly attachedin place. Once attached, the head clamp can be secured to an operatingtable. Various examples of such head clamps are described in U.S. Pat.No. 4,169,478, U.S. Pat. No. 5,318,509, U.S. Pat. No. 6,381,783 and U.S.Pat. No. 7,229,451.

In stereotactic neurosurgery, a neurosurgeon will often have to insertinstruments into the brain with millimetre accuracy. In particular, thesuccess of stereotactic neurosurgery is highly dependent on the accuracywith which neurosurgical instruments, such as electrodes or catheters,can be guided to a predetermined target site within the brain.Stereotactic head frames for use in precision guided neurosurgery areknown. For example, the Leksell (Registered Trade mark) frame producedby Elekta AB, Sweden includes a base ring attachable to the base of theskull. Once the base ring is attached to the subject's head and has beenimaged it acts as a platform of known position relative to the head fromwhich instruments can be stereotactically guided to the required targetsite or sites in the brain. The base ring can also be locked to anoperating table to immobilise the head. It has also been proposedpreviously to mount MRI coils directly to the base ring of a Lekselltype frame. For example, WO2006/134357 describes apparatus in which RFcoils for MRI can be directly attached to such a base ring.

According to a first aspect of the present invention, a head clamp forneurosurgery is provided that comprises a member for at least partiallyencircling the head of a subject, and at least first and second skullattachment portions for attaching the member to the head of a subject,characterised in that the head clamp comprises a position setter thatallows the member to be moved between at least two repeatable relativepositions.

The present invention thus provides a head clamp for neurosurgerycomprising a member that is attachable to a head using first and secondskull attachment portions that engage the skull of a subject. The membermay be of any shape but it is preferred that the member comprises ac-shaped member. Advantageously, such a c-shaped member partiallyencircles the head. It should be noted that the term c-shaped member isused herein to encompass members that may be v-shaped, u-shaped,G-shaped, arced, bent or a variant shape thereof. Conveniently, thefirst and second skull attachment portions are located at the first andsecond ends of the c-shaped member. Advantageously, the first and secondskull attachment portions are locatable at substantially opposite sidesof a skull. As described in more detail below, the skull attachmentportions may include one or more bone pins or screws that can be secureddirectly to the skull bone of a subject. A force applicator may also beprovided, e.g. as described below, to force such pins into contact withthe skull. Means may also be provided to prevent rotation of the firstand/or second skull attachment portions relative to the member duringthe attachment or removal of the head clamp.

The present invention also includes a position setter that allows themember to be moved between at least two repeatable relative positions.The position setter may comprise a position encoder (e.g. a rotaryencoder) or positional markings that measure, or allow to be measured,an angle between the member and a part of the head clamp that has aninvariant position relative to the skull to which it is attached.Advantageously, the position setter comprises an indexing mechanism thatallows, when the head clamp is attached to a head, the member to beindexed between two or more repeatable relative positions.

Providing a head clamp with a position setter in accordance with thepresent invention allows the member to be moved between at least tworepeatable positions that have a known (predetermined) position relativeto each other. This allows, for example, measurements (e.g. MRI images)of the head that are tied to a unique reference or datum position on themember when that member is in a first position to be linked back to thatreference position when the member has been moved into the secondposition. In this manner, MRI images of the brain acquired with themember in a first position can be used, after applying appropriateco-ordinate transformations, to precisely guide neurosurgical deviceseven when the member has been moved into another position. This abilityto move the member into a plurality of repeatable relative positions isparticularly advantageous because it enables the member to be movedbetween different positions that allow optimised imaging (e.g. usingMRI) or provide the necessary access to the head for the insertion ofneurosurgery devices or instruments. Advantageously, the position setterallows the member to be moved whilst the clamping force for holding thehead clamp on the head is applied.

None of the known head clamp arrangements mentioned above provide aposition setter according to the present invention. The c-shaped membersof known head clamps can be finely adjusted during attachment to thehead, but are not moveable between two or more repeatable relativepositions. Such known head clamps are simply adjusted into a preferredposition and then used as a means for securing the head to an operatingtable during neurosurgery. Similarly, the base ring of a Leksell frameis secured in position and then used as a reference platform after beingimaged; the base ring is therefore specifically designed to be asimmobile as possible after attachment to the skull. In contrast to theseprior art devices, the present invention offers, by virtue of theprovision of a position setter such as an indexing mechanism, theability to optimise the position of the member relative to the head fordifferent applications without loosing the ability to then use anypositional information that has been obtained using the member as areference or base position.

Advantageously, the member can, when the head clamp is attached to thehead of a subject, be moved (e.g. indexed) into a first position.Conveniently, the first position comprises a position in which themember substantially extends around the top of the head (e.g. in aso-called sagittal position) thereby allowing imaging (e.g. MRI) of thehead. In such a first position, MRI coils and/or MRI visible fiducialmarkers may be attached (directly or indirectly) to the member.Orientating the member so that it substantially extends around the topof the head allows such fiducial markers and associated RF coils to beplaced in close proximity to the side of the head and allows such coilsto extend along the whole side of the head thereby allowing more of thehead to be imaged. In other words, the first position of the member maybe selected to optimise imaging.

Preferably, the member can, when attached to the head of a subject, bemoved (e.g. indexed) into a second position. Advantageously, the secondposition comprises a position in which the member substantially extendsaround a side of the head thereby enabling neurosurgical procedures(e.g. stereotactic neurosurgery) to be performed on the head. Such asecond position thus allows the member to be moved away from the top ofthe head to one side of the head. This allows the surgeon to haveimproved access, substantially unhindered by the head clamp, to therequired part of the head through which the brain will be accessed. Inother words, the second position of the member may be selected tooptimise access for neurosurgery.

The first and second positions into which the member can be moved arerepeatable relative positions. In other words, the relative differenceor change in position of the member between the first and secondpositions is known. For example, the head clamp may be designed suchthat there is a certain positional difference between the first andsecond positions and/or any positional difference may be measured.Knowing the relative difference in the first and second positions meansthat any position measurements made relative to the member with the headclamp in the first position (e.g. using MRI) can be tied to positions onthat member with the head clamp in the second position. In this manner,neurosurgical instruments guided by a stereoguide attached to the memberof the head clamp may be accurately guided to target sites in the brainidentified using MRI, even though the member may be indexed intodifferent positions during imaging and surgery.

The head clamp may be attached to a skull by only the first and secondskull attachment portions. Alternatively, the head clamp may includefurther skull attachment portions. Advantageously, the first and secondskull attachment portions are locatable on substantially opposite sidesof a head. Preferably, the first and second skull attachment portionsexert a clamping force on the head substantially along a clamping axis.The head clamp, when attached to the head, is conveniently arranged sothat the clamping axis is substantially parallel to the anteriorcommisure posterior commisure (ACPC) plane of the brain. Alignment withthe ACPC, which is an internal landmark of the brain, may be achievedsufficiently accurately by aligning the clamping axis with externallandmarks such as the cantho-meatal plane. The first skull attachmentportion is, in use, conveniently attached to the forehead of thesubject.

The member (e.g. a c-shaped member) is advantageously rotatable aboutthe clamping axis. For example, the member may be moveable (e.g.indexable) between two or more rotary positions. The member may thusadopt a first position in which the member substantially extends aroundthe top of the head; this may be defined as a 0° position. A secondposition in which in which the member substantially extends around aside of the head may then be defined as a +90° or a −90° position(depending on which side of the head it is positioned). Preferably, aplurality of repeatable positions between −135° and 135° are provided.The position setter may, of course, provide other repeatable relativeangular positions if required.

In order to provide the positional link between different positions, itis preferable that the user is able to unambiguously establish theparticular repeatable relative position into which the member has beenplaced. This may be achieved by providing markings or the like on thehead clamp that allows the selected position to readily determined. Ifan indexing mechanism is provided, it is also preferred that the membercan only be indexed into relatively few different positions therebyensuring there is no confusion over the position in which the member islocated. Preferably, such an indexing mechanism allows the member to beindexed in to fewer than twenty, more preferably fewer than ten, morepreferably fewer than five, or more preferably three or fewer, or morepreferably fewer than three different repeatable relative positions. Ifthe member is rotatable about an axis as described above, it ispreferred that the repeatable relative positions are separated by anangle of at least 5°, more preferably at least 10°, more preferably atleast °, more preferably at least 45° and more preferably at least 75°.

The position setter may comprise an indexing mechanism of any suitabletype. Conveniently, the indexing mechanism allows the member to beindexed when the clamping force for engaging the head clamp with theskull (e.g. as produced by the force applicator) is being applied.Preferably, each indexed position provided by such an indexing mechanismcan be selected with no, or negligible, backlash. Advantageously, theindexing mechanism comprises first and second mating parts that mayadopt a plurality of different, repeatable, indexed positions relativeto one another. The indexing mechanism may comprise parts that provide av-tooth, Curvic or Hirth coupling. Alternatively, the first part maycomprise a plurality of pins that mate with a plurality of tapered holesprovided in the second part. In a further alternative embodiment, eachpart may comprise a set of balls arranged to engage each other to definethe plurality of indexed positions. Each indexed position may beprovided by a kinematic connection. Advantageously, the first matingpart of the indexing mechanism may be rotatable relative to a secondmating part of that mechanism; e.g. the first mating part may be locatedat least partially within the second mating part. A locking screw may beprovided to lock the first and second parts into the selected indexedposition. The locking screw may engage a feature of the first part orsecond part.

The first and second skull attachment portions may include any suitableelements for engaging the head. Advantageously, any such elements passthrough openings in the skin and directly engage with the skull bone ofthe subject. The first skull attachment portion and/or the second skullattachment portion may conveniently comprise one or more pins for directengagement with the skull bone of a subject. Advantageously, the firstskull attachment portion comprises the same number of pins as the secondskull attachment portion. Preferably, the first and second skullattachment portions each include two pins for direct engagement with theskull bone of a subject. Such an arrangement, in particular providingthe first and second skull attachment portions with the same number ofpins, is preferred as it allows the member to be loaded with therequired clamping force without being substantially distorted ortwisted.

Advantageously, each pin has an outer, soft, coating. For example, metal(e.g. titanium) pins may be provided that are coated with an outer layerof soft material (e.g. rubber, plastic etc). Such a coating providesprotection against damage to soft tissue during attachment of the headclamp to a subject. It should be noted that a bone attachment pin ofthis type may also be used in different applications. A pin is thusdescribed herein that comprises a core (e.g. a metal or ceramic core)with a sharp tip, wherein a soft coating is provided on the tip.

The first skull attachment portion may conveniently comprise a pincarrying member that is pivotally attached to the member. For example,the pin carrying member may be pivotally attached to the first end of ac-shaped member. The second skull attachment portion may also comprise apin carrying member that is pivotally attached to the member. Forexample, the pin carrying member may be pivotally attached to the secondend of a c-shaped member. Each pin carrying member may carry, or bearranged to carry, one or more pins for direct engagement with the skullbone of a subject. Attaching pins to the end of the c-shaped memberusing such a pivotally attached pin carrying member allows the appliedclamping force to be evenly distributed between the pins and minimisesout of plane distortions of the member. Advantageously, each pincarrying member is arranged to pivot relative to the member in a singleplane.

The head clamp may comprise a force applicator to urge the first andsecond skull attachment portions into engagement with the skull. Theforce applicator may urge the first and second skull attachment portionsinto engagement with the skull with a variable, e.g. user definable,force. Preferably, the force applicator urges the first and second skullattachment portions into engagement with the skull with a predeterminedamount of force. For example, the force applicator may be arranged toapply a certain force (e.g. 200 N) to the skull per skull attachmentpin. Advantageously, the force applicator may include a force indicatorfor indicating when a preferred engagement force has been applied. Theforce indicator preferably comprises a tactile indicator, such as arecessed rod that becomes flush with a reference surface when therequired force is applied. The force applicator may also comprise aforce limiter for ensuring that a certain force limit is not exceeded.For example, the force limiter may ensure the predetermined amount offorce is not exceeded.

The force applicator may be located adjacent to only one of the firstand second skull attachment portions. For example, the force applicatormay be located at only one end of a c-shaped member (e.g. at theopposite end to the position setter or indexing mechanism). This,however, obviously results in the application of an equal and oppositeforce at the first and second ends of the c-shaped member. It can alsobe seen that the clamping force is transmitted between the skullattachment portions via the member. Applying a predetermined clampingforce in this manner also ensures that the member is, in use, subjectedto the same (predetermined) force and hence deforms, at leastapproximately, by a known amount. The member does not, therefore, haveto be sufficiently rigid to be undistorted by the clamping force.Instead, it is preferred that the member deforms by at leastapproximately the same amount when the predetermined clamping force istransmitted through it.

The member preferably comprises at least one datum feature. Each suchdatum feature preferably provides a positional reference point on or ina fixed positional relationship to the member. Each datum feature may bea marking (e.g. a visible or MRI visible marking) or a physical featureor set of features. A single datum feature may be provided.Advantageously, a datum feature is provided on each side (e.g. onopposing faces) of the member.

Advantageously, the at least one datum feature is located substantiallyon a neutral axis of distortion of the member; the neutral axis being anaxis about which the member deforms when the clamping force is applied.If two datum features are provided on opposite sides or faces of themember, each datum feature may be located substantially on a neutralaxis of distortion of the member. Any variations in the applied clampingforce will thus have no substantial effect on the position of the datumfeature(s) relative to the first and second skull attachment portions orthe head to which the clamp is attached. Associated apparatus may theninclude a feature that is complementary to the datum feature such thatthe associated apparatus may be repeatedly placed in the same definedposition relative to the datum feature of the head clamp.

Advantageously, a datum feature is provided on the member that comprisesthe first part of a kinematic joint. In other words, the datum featuremay comprise a kinematic datum feature. A kinematic datum feature maythus be arranged so that it uniquely constrains an associated,complementary, kinematic feature in each of the 6 degrees of freedom.For example, the datum feature may comprise three v-grooves radiallyspaced apart from one another by 120° and extending along directionsthat intersect at a common point. In such an example, the common pointmay be located substantially on the neutral axis of the member.Associated apparatus (e.g. stereoguides, fiducial markers for MRI etc)may then comprise the second part of the kinematic joint, such as threeballs (or at least partly spherical features) spaced in a circle andseparated from each other by 120°. The first and second parts of thekinematic joint may then be arranged to provide a highly repeatablekinematic link in which there is only one constraint on each degree offreedom of movement between the first and second parts.

Advantageously, the head clamp comprises at least one attachment featurethat allows the head clamp to be secured to associated apparatus.Preferably, the at least one attachment feature is located substantiallyon a neutral axis of distortion of the c-shaped member. The at least oneattachment feature may thus provide a mechanical linkage by which thehead clamp can be secured to associated apparatus such as an operatingtable bed or an MRI headcoil assembly. A common feature may provide botha datum feature and an attachment feature. Advantageously, theattachment feature is separate from the datum feature. Preferably, adatum feature of the head clamp is accessible when the attachmentfeature is secured to the associated apparatus. This allows, forexample, the at least one attachment feature to be used to secure thehead clamp to an operating table bed whilst a feature of a stereoguideis mated with the datum feature of the head clamp. Supplementaryattachment features may also be provided on the member if required.

Advantageously, the head clamp is substantially MRI compatible. Forexample, the member may be formed from a polymer, a reinforced plastic,a filled ceramic or a glass-filled polymer. The head clamp may alsoinclude ceramic and/or plastic components. The head clamp may containsome metal components (e.g. springs), but the amount of metal ispreferably not enough to cause the generation of eddy currents that aresufficient to cause substantial heating of the structure or inducemagnetic field distortions that effect the MR image.

The present invention also provides a head clamping kit that comprises ahead clamp as described above and a secondary clamping device that isattachable to the head clamp. Advantageously, the secondary clampingdevice provides, when attached to the head clamp, additional mechanicalsupport to the member. Conveniently, the secondary clamping deviceprovides such mechanical support to the member only in the event ofmechanical failure of that member. In other words, the secondary clampis normally non-load bearing and only provides mechanical support if themember fails. This backup or support clamp may be used during surgicalprocedures in which the force exerted on the head clamp may momentarilyexceed safe tolerances.

Also described herein is a kit comprising a head clamp for neurosurgeryand a secondary clamping device attachable to the head clamp. The headclamp may comprise a member for at least partially encircling the headof a subject. For example, the head clamp may comprise a c-shaped memberfor partially encircling the head of a subject. The head clamp mayinclude first and second skull attachment portions; for example providedat the first and second ends of a c-shaped member. The secondaryclamping device provides, when attached to the head clamp, additionalmechanical support to the member (e.g. the c-shaped member) in the eventof mechanical failure of that member.

The present invention also extends to apparatus for imaging the head ofa subject that comprises a head clamp as described above and apparatus(e.g. MRI or CT apparatus) for imaging a head. Preferably, the headclamp is releasably attachable to the apparatus. For example, the MRIapparatus may include a clamping mechanism for engaging an attachmentfeature of the head clamp and/or one or more fiducial markers that canbe repeatably attached to a datum feature of the head clamp.

The apparatus for imaging the head of a subject may comprise a housingfor at least partially surrounding a body part, and a first fiducialmarker assembly retained at least partially within the housing thatcomprises one or more fiducial markers and a datum feature, the positionof the datum feature being fixed relative to the one or more fiducialmarkers, wherein the first fiducial marker assembly is moveable withrespect to the housing and the datum feature is accessible from outsideof the housing. Such apparatus may include one or more RF coils formagnetic resonance imaging that are located within the housing. Thehousing may comprise a first housing part and a second housing part,wherein the first and second housing parts can be moved, e.g. pivoted,into a closed position that defines an imaging space in which a humanhead can be located. Fiducial marker assemblies may also be provided ineach housing part. Such apparatus is described in more detail inApplicant's co-pending PCT patent application, the contents of which arehereby incorporated by reference, that has the same filing date as thepresent application and claims priority from UK patent application No.0908784.

The present invention also extends to apparatus for neurosurgery,comprising a head clamp as described above and a stereoguide device forretaining and guiding neurosurgical instruments. Preferably, thestereoguide device is releasably attachable to the head clamp. Forexample, the neurosurgery apparatus may comprise a clamp for engaging anattachment feature of the head clamp and/or the stereoguide device maybe repeatably attached to a datum feature of the head clamp. The memberof the head clamp may be moveable into repeatable relative positionsthat provide an axis of movement or rotation of the stereoguide.

According to a second aspect of the invention, a method of performingneurosurgery is provided that comprises the steps of (i) attaching anhead clamp comprising a moveable member to the head of a subject, (ii)moving the member into a first position and acquiring at least one imageof the head, (iii) moving the member into a second position andperforming a neurosurgical procedure, wherein the relative difference inposition between the first position and the second position of themember is known.

Preferably, the head clamp comprises an indexable member and steps (ii)and (iii) comprise indexing the member into the first position andsecond position respectively. Advantageously, at least one datum featureis provided on the head clamp that allows images acquired during step(ii) to be related to neurosurgical procedures conducted during step(iii). Preferably, step (i) comprises attaching the head clamp to thehead with a predetermined engagement force such that deformations of themember are known. Conveniently, step (i) comprises applying a clampingforce substantially along a clamping axis, wherein the clamping axis isaligned, at least approximately, with the cantho-meatal plane of thesubject.

Preferably, the member of the head clamp comprises a c-shaped member,wherein the first position of step (ii) is a position in which thec-shaped member substantially extends around the top of the head therebyallowing imaging of the head and the second position of step (iii) is aposition in which the c-shaped member substantially extends around aside of the head thereby providing access the head to allow theneurosurgical procedure to be performed.

Also described herein is a head clamp for neurosurgery, comprising; ac-shaped member for partially encircling the head of a subject, andfirst and second skull attachment portions provided at first and secondends of the c-shaped member, wherein the head clamp comprises at leastone fiducial marker (e.g. at least one MRI visible fiducial marker). Atleast one fiducial marker may be formed integrally with the head clamp.For example, a fiducial marker may form part of, or be fixed to, anypart of the head clamp. At least one fiducial marker may be attachable(e.g. releasably attachable) to a part of the head clamp in a knownposition relative to that part of the head clamp. The head clamp mayalso comprise a datum feature (e.g. provided on the c-shaped member)that allows associated apparatus to be located in a known positionrelative to the head clamp. Such a datum feature may also allow thefiducial marker to be attached to the head clamp in a known relativeposition.

A fiducial assembly may also be provided that comprises at least onefiducial marker (e.g. at least one MRI visible fiducial marker) and adatum feature having a fixed position relative to the at least onefiducial marker. The datum feature may be combined with one or morefiducial markers. The datum feature preferably allow associatedapparatus to be located in a known position relative to the assembly.The fiducial assembly may include means for attachment to the skull of asubject. For example, such means may comprise a clamp (e.g. a c-clamp)for securing the fiducial assembly to a skull.

The invention will now be described, by way of example only, withreference to the accompanying drawings in which;

FIG. 1 illustrates a head clamp of the present invention,

FIG. 2 shows an exploded view of the force control mechanism of the headclamp of FIG. 1,

FIG. 3 is a view of the force control mechanism prior to the requiredforce being applied,

FIG. 4 is a view of the force control mechanism when the required forceis being applied,

FIG. 5 is an exploded view of the indexing mechanism of the head clampof FIG. 1,

FIG. 6 illustrates a rubber coated attachment pin,

FIG. 7 shows the kinematic datum feature of the head clamp in moredetail,

FIG. 8 shows a frontal view of the head clamp when attached to a headand indexed into a first (imaging) configuration,

FIG. 9 shows a side view of the head clamp when attached to a head andindexed into a first (imaging) configuration,

FIG. 10 shows a frontal view of the head clamp when attached to a headand indexed into a second (surgical) configuration,

FIG. 11 shows a side view of the head clamp when attached to a head andindexed into a second (surgical) configuration,

FIG. 12 shows MRI imaging apparatus for receiving a head clamp of thetype shown in FIGS. 1 to 11 in an open configuration,

FIG. 13 shows a head clamp retained by the MRI apparatus,

FIG. 14 shows the MRI imaging apparatus in a closed configuration,

FIG. 15 shows one part of the housing of the MRI imaging apparatus inmore detail,

FIG. 16 provides a more detailed view of how the fiducial markerassembly is attached to the housing,

FIG. 17 provides an exploded view of how the fiducial marker assembly isattached to the housing,

FIG. 18 shows an optional support or back-up clamp for the head clamp,

FIG. 19 is an exploded view of a Hirth coupling that can provide analternative indexing mechanism for the head clamp of FIG. 1,

FIG. 20 shows the Hirth coupling of FIG. 19 in an unlockedconfiguration,

FIG. 21 shows the Hirth coupling of FIG. 19 in a locked configuration,

FIG. 22 shows an anti-rotation device for locking the force applicatormechanism of a head clamp as shown in FIG. 1,

FIG. 23 shows the anti-rotation device of FIG. 22 in an openconfiguration when engaging the head clamp, and

FIG. 24 shows the anti-rotation device of FIG. 22 in a closedconfiguration that prevents rotation of the force applicator mechanism.

Referring to FIG. 1, a head clamp 2 of the present invention isillustrated. The head clamp 2 comprises a generally c-shaped member 4having a first end 6 and a second end 8.

At the first end 6 of the c-shaped member 4, a first pair of skullattachment pins 10 and 12 are mounted to a first v-shaped pin carryingmember 14. The first v-shaped member 14 is pivotally connected at itsapex to a force applicator mechanism 16 which is in turn attached to thefirst end 6 of the c-shaped member 4. The first v-shaped member 14 ispivotable about the axis P1 such that the pins 10 and 12 can rotate in asingle plane. The force applicator mechanism 16 comprises a profiledportion 18 which can be rotated to drive the first v-shaped pin carryingmember 14 along the clamping force axis 20 thereby allowing the headclamp to be secured to the head of a subject. More details of the forceapplicator mechanism 16 are outlined below with reference to FIG. 2.

At the second end 8 of the c-shaped member 4 there is provided a secondpair of skull attachment pins 22 and 24 that are mounted to a secondv-shaped pin carrying member 26. The second v-shaped member 26 ispivotally connected to an indexing mechanism 28 that is in turn mountedto an aperture provided at the second end 8 of the c-shaped member 4.The second v-shaped member 26 is pivotable about the axis P2 such thatthe pins 22 and 24 can rotate in a single plane. The indexing mechanism28 comprises a locking screw 29 for locking the selected indexedposition. More details of the indexing mechanism 28 are provided belowwith reference to FIG. 3.

The c-shaped member 4 is approximately mechanically symmetrical aboutthe plane 30 which provides a neutral axis of distortion. A neutral axisof distortion is thus the axis of symmetry of the c-shaped member 4 thatis not subject to compression or tension (i.e. is not distorted) whenthe head clamp exerts a clamping force on a head. The c-shaped member 4also comprises ribs 31 to improve stiffness and in particular to reducethe possibility of the c-shaped member 4 twisting. In this example, thec-shaped member 4 is formed from a glass-filled polymer material, suchas Ryton R7, which is light, stiff and MRI compatible. It should benoted that it would also be possible to use other materials (e.g.ceramic) to form the c-shaped member. For example, a number of differentpolymer materials could be used and it would also be possible to use aferrous metal (such as steel) if MRI compatibility was not required.

On both faces of the c-shaped member 4 there are provided identicaldatum features that lie on a neutral axis. The datum feature shown inFIG. 1 comprises three sub-features in the form of v-grooves 32 a-32 cthat are radially spaced apart from one another by 120° and extend alongdirections that intersect at a common point. As explained in more detailbelow, the v-grooves 32 a-32 c of the datum feature provide arepeatable, kinematic, mechanical link with associated apparatus such asa stereoguide or MRI fiducial marker assemblies. The c-shaped member 4also comprises an attachment feature on each face. The attachmentfeature is, in this example, provided in the form of three recesses 34a-34 c and a circular rim portion 35.

The three recesses 34 a-34 c and/or the circular rim portion 35 can beused to provide a pseudo-kinematic link that allows the head clamp to besecured to apparatus such as a surgical bed or MRI apparatus in anapproximate position. In this example, attachment features are providedon both faces that are co-axial with the datum features and thereforealso lie substantially on the neutral axis. It would, of course, also bepossible to space apart the attachment and datum features and/or provideeither feature on only one face. In this example, supplementaryattachment features are also provided by the protrusion 37 and theapertures 38 provided in the c-shaped frame 4. Apparatus may be securedto the head clamp 2 using some or all of the attachment features andsupplementary attachment features as appropriate; this is described inmore detail below.

More details of the various parts of the head clamp of FIG. 1 will nowbe described with reference to FIGS. 2 to 7.

Referring to FIG. 2, the force applicator mechanism 16 is illustrated inan exploded view. The force applicator mechanism 16 comprises a forcecontrol member 40 that comprises the profiled portion 18 at its proximalend, a force indication rod 42, a helical spring 44, a hollowcylindrical sleeve 48 that fits within the aperture at the first end 6of the c-shaped member 4 and a shaft portion 50. An optional clampingelement 46 may also be provided to lock the force control member 40 tothe first end 6 of the c-shaped member 4. The distal end of the shaftportion 50 comprises a yoke 52 to which the v-shaped pin carrying member14 can be attached using a pair of pivot bolts 56 and 58 and a threadedloading stud 54. Skull attachment pins 10 and 12 can be attached to thetapered apertures of the v-shaped pin carrying member 14.

The force applicator mechanism 16, when assembled, allows apredetermined force to be applied to the skull via the skull attachmentpins 10 and 12. In particular, rotation of the force control member 40causes the spring 44 to urge the shaft portion 50 along the clampingaxis 20 thereby moving the pins 10 and 12 into engagement with theskull. As the clamping force applied to the skull is increased, thespring 44 compresses which in turn causes the force indication rod 42 tomove relative to the force control member 40. The face 60 of the forceindication rod 42 become flush with the face 62 of the profiled portion18 when the required, preset, clamping force is being applied. Althoughnot shown, an optional force limiter may be provided to prevent thepreset clamping force being exceeded.

Referring now to FIG. 3, the tactile indicator of the force applicatormechanism 16 is shown before the predetermined clamping force isapplied. It can be seen that the face 60 of the force indication rod 42is sub-flush or recessed with respect to the face 62 of the profiledportion 18 of the force control member 40.

Referring to FIG. 4, the tactile indicator of the force applicatormechanism 16 is shown when the required, predetermined, clamping forceis applied. It can be seen that the face 60 of the force indication rod42 is flush with respect to the face 62 of the profiled portion 18 ofthe force control member 40. This provides the user with a simple,tactile, indication that the desired clamping force is being applied.

Referring to FIG. 5, the indexing mechanism 28 is shown in an explodedview with insets providing alternative views of several components. Theindexing mechanism comprises a detent insert 70, an indexer body ring72, a locking or indexing screw 29 and an indexer-head shaft 74. Thedistal end of the indexer-head shaft 74 comprises a yoke 84 to which thev-shaped pin carrying member 26 can be attached using a pair of pivotbolts 88 and 90 and threaded loading stud 86. Skull attachment pins 22and 24 can be attached to tapered apertures of the v-shaped pin carryingmember 26.

The indexer-body ring 72, when assembled, is moulded into an indexer-eye73 provided at the inwardly facing side of the aperture provided at thesecond end 8 of the c-shaped member 4. The proximal end 76 of theindexer-head shaft 74 can, when the locking screw 29 is unscrewed,freely rotate within the indexer body ring 72. The indexing mechanism isconfigured to operate when the head frame is clamped to the head andhence, in use, there will be a force of around 400 N urging theindexer-head shaft 74 into engagement with the indexer-body ring 72.

The locking screw 29 has a conical tip 79 and the indexer-head shaft 74comprises a chamfered collar 80 in which are formed a plurality ofconical depressions 75. The indexer-body ring 72 comprises threechamfered buttresses. The required indexed position is locked when theconical tip 79 of the indexer screw 29 emerges through the hole 73 inthe indexer-body ring 72 and engages with one of the conical depressions75 of the indexer-head shaft 74. When the screw 29 is fully engaged witha conical depression 75, the indexer-head shaft 74 is displacedfractionally away from its rotation axis so that its chamfered collar 80is pushed into and supported by two chamfered buttresses 78 of theindexer-body ring 72. This three point support provides backlash freeand accurate positioning.

The detent insert 70 is a moulded thermoplastic component that is heldin place in the outwardly facing side of the aperture provided at thesecond end 8 of the c-shaped member 4 by a pair of sprung lugs 71. Thepurpose of the detent insert 70 is to guide the indexer-head shaft 74into angular positions relative to the indexer-body ring 72 that areclose enough to the locked or indexed positions to enable the indexerscrew 29 to engage the selected one of the conical depressions 75. Thedetent insert 70 also forms the function of providing tactile feedbackto a user that an indexed position has been achieved by clicking intoplace. The detent insert 70 comprises four protruding sprung elements 73that provide this positional click by engaging with complementary slots81 located at the index positions that are provided at the proximal end76 of the indexer-head shaft 74.

The indexer-head shaft 74 is made in a single piece. This is notessential but has been found to aid positioning between the indexingcomponents (e.g. the chamfered collar 80 and conical depressions 75) andthe yoke 84 whilst also simplifying assembly and cleaning. Assembly ofthe indexer-head shaft 74 and the indexer-body ring 72 is made possibleby the inclusion of slots 82 in the chamfered collar 80 which can bealigned with and pass over the chamfered buttresses 78.

The indexing mechanism 28 thus allows a plurality of discrete andrepeatable indexed positions to be provided. It should, however, benoted alternative indexing mechanisms could be used to provide the samefunction. For example, the indexing mechanism may be implemented using av-tooth (e.g. Curvic or Hirth) coupling, a plurality of pins that matewith a plurality of tapered holes or complementary sets of balls.Instead of the indexing mechanism of the present example, differenttypes of position setter could be provided; for example, a rotaryposition encoder could be provided instead of the indexing mechanism.

Referring to FIG. 6, a pin 100 for engaging the skull of a subject isshown. Such a pin may be used in apparatus as described above or as partof any other appropriate surgical apparatus. The pin 100 comprises atitanium core having a tapered portion 122 (e.g. that can be pushed intothe tapered bore on the arm of the v-shaped member) and a sharp skullengaging tip 124. Although a tapered connection between the pin andmember is shown, other connections (e.g. threaded connections) may beprovided. The skull engaging tip 124 is coated with a ball of a soft,e.g. rubber, material 126. This soft coating or material 126 helpsprevent unwanted damage to soft tissue (e.g. skin) whilst a head clampis being positioned but it can be readily pierced by the titanium tipwhen the pins are located in the required position and a clamping forceis applied. It should be noted that the soft coating, althoughpreferred, is by no means essential. Similarly, the pins may be formedfrom many materials other than titanium (e.g. other metals or ceramicetc). The pin 100 may be permanently attached to a part of the headclamp but it is preferably provided as disposable (e.g. single use) itemthat can be easily attached to and detached from the head clamp.

Referring to FIG. 7, the datum and attachment features of the head clampare shown in more detail. The datum feature comprises three v-grooves 32a-32 c that are radially spaced apart from one another by 120°. Theattachment feature comprises three recesses 34 a-34 c and the circularrim portion 35. Apertures 38 that are provided as part of thesupplementary attachment features are also shown.

A number of potential uses of the head clamp described with reference toFIGS. 1 to 7, along with methods of attaching the head clamp to asubject, will now be described with reference to FIGS. 8 to 11.

In use, the head clamp 2 is placed over a subject's head so that thec-shaped member 4 partially encircles the head. The pins 10 and 12provided at the first end 6 of the c-shaped member 4 are aligned withthe centre of the forehead and the pins 22 and 24 provided at the secondend 8 of the c-shaped member 4 are located on the opposite side (rear)of the head. The clamping axis 20 of the head clamp is alsoapproximately aligned with the cantho-meatal plane. An alignment aid,for example a band attached to the head or an alignment aid mounted tothe head clamp, may be used to facilitate the required alignment. Thefirst and second v-shaped members 14 and 26 are also respectivelyarranged such that the pins 10 and 12 pivot in substantially the sameplane as the pins 22 and 24. Once aligned, the profiled portion 18 ofthe force applicator mechanism 16 is turned thereby advancing the pins10 and 12 towards the head. The applied force is then increased untilthe pins 10, 12, 22 and 24 cut through the skin and engage theunderlying skull bone with the required, predetermined, amount of force.In this example, a total force of 400 N is applied which results in eachpin applying a 200 N force to the skull bone. Pivoting of the first andsecond v-shaped members 14 and 26 ensures that the force is evenlyapplied to the skull through each pin.

Applying a predetermined (preset) amount of force evenly distributedthrough the pins has the advantage that the c-shaped member 4 willdeform by substantially the same amount each time it is attached to ahead. Furthermore, locating the datum and attachment featuressubstantially on the neutral axis ensures that the position of suchfeatures is substantially invariant even if the c-shaped member issubjected to slightly different distortion forces.

FIGS. 8 and 9 show the head clamp 2 attached to a head 140 and indexedinto a first, imaging, position. In this first position, the c-shapedmember extends around the top of the head 140. This position isparticularly suited for imaging applications, such as MRI, because itallows MRI coils and/or fiducial markers to be placed in close proximityto both sides of the head without any interference from the c-shapedmember 4.

FIGS. 10 and 11 shows the head clamp 2 attached to the head 140 andindexed into a second position in which the c-shaped member extendsaround the side of the head. This second position provides the access tothe top of the head that a surgeon typically requires to performstereotactic neurosurgery.

Although two different indexed positions are shown in FIGS. 8-11, itshould be noted that the c-shaped member of the head clamp could beindexable between more positions. For example, the c-shaped member couldalso be indexable into a position on the other side of the head and/orinto any one or more other (e.g. intermediate) indexed positions. It is,however, preferably to provide only a few index positions to ensure thatthere is no potential for confusion over the index position that hasbeen set. Markings may also be provided, if required, to help indicatethe indexed position that has been adopted.

The positional differences between the various indexed positions adoptedby the head clamp are preferably known or measured. For example, thehead clamp may be designed so that there is a predetermined positionalchange in the position of one or more datum features between each of theindexed positions. Alternatively, a calibration procedure may beperformed prior to use (e.g. during manufacture of the head clamp) inwhich the position of the datum feature is measured for differentindexed positions. The head clamp may thus be supplied with a set ofco-ordinate transformations that provide such position mappinginformation.

It should be noted that the head clamp may be formed from the glassfilled polymer material mentioned above or any other suitablealternative material(s). The various parts of the head clamp may besingle use (disposable), multi-use or re-useable. If any part of thehead clamp can be used more than once, it is preferred that such a partcan be sterilised in an autoclave.

Referring now to FIGS. 12 to 17, there will be described apparatus forimaging the head of subject that is designed for use with a head clampof the type described with reference to FIGS. 1 to 11. Although theapparatus may be used with any imaging technique, the following examplesdescribe its use with MRI apparatus.

FIG. 12 illustrates head imaging apparatus suitable for use in MRI thatis placed in its open position. The apparatus comprises a housing formedfrom a first housing part 160 that is connected to a second housing part162 by a pivot joint 164. As will be described in more detail withreference to FIG. 14-16 below, each housing part comprises a hollowplastic shell that contains RF coil assemblies for MRI, variouselectronic control circuitry and a floating fiducial marker assembly.The fiducial marker assemblies of the first and second housing parts areretained in their respective housing parts and have inwardly facingdatum features 166 and 168 that are externally accessible through anaperture formed in the housing. A clamp mechanism in the form of atoggle clamp 170 is provided on the base 171 of the apparatus betweenthe two housing parts 160 and 162.

As illustrated in FIG. 13, the toggle clamp 170 is designed such thatactivation of lever 172 causes the clamp to engage and hold the circularrim portions 35 of the two attachment features provided on the opposedfaces of the c-shaped member 4 of the head clamp 2 that is describedabove with reference to FIGS. 1 to 11. The head imaging apparatus alsocomprises a channel 173 having angled walls for engaging the protrusion37 of the c-shaped member 4 to provide additional mechanical support.The toggle clamp 170 is also arranged such that, when engaging thecircular rim portions 35 of the head clamp 2, the datum features of thehead clamp 2 are still accessible. In particular, the datum features 166and 168 of the fiducial marker assemblies are arranged such that theycan be brought into contact with the complementary datum features of thehead clamp when held by the toggle clamp 170.

FIG. 14 illustrates the apparatus described above with reference toFIGS. 12 and 13 in its closed position. A two-part locking latch 174 isprovided to hold the housing parts together in the closed position. Inthis closed position, the datum features 166 and 168 of the fiducialmarker assemblies are biased into engagement with the complementarydatum features of the head clamp 2. The fiducial markers of the fiducialmarker assemblies are thus accurately held in a known, repeatable,position relative to the datum features of the head clamp. Thesefiducial markers thus act as highly accurate and repeatable referenceposition markers in any acquired MRI images.

Referring to FIG. 15, the components located within the first housingpart 160 of the apparatus described with reference to FIGS. 12 to 14will be described. It should be noted that the second housing part 162houses similar components and is therefore not shown.

The first housing part 160 comprises a plastic shell that holds thevarious sets of RF coils 180 that can be used in conjunction with MRIapparatus to obtain high resolution images of the head. These RF coils180 are secured to the housing and connected to electronic controlcircuitry provided on circuit boards 182. The RF coils 180 and circuitry182 typically handle electrical signals during use and the first housingpart therefore also acts as an insulating shield that prevents patientsand operators being exposed to the electrical voltages. Cables to andfrom the electrical circuitry 182 are routed via the pivot joint of thehousing.

The first housing part 160 also contains a first floating fiducialmarker assembly 190. The fiducial marker assembly 190 comprises afiducial marker 192 in the form of a square frame 194 with a diagonalcross member 196. The fiducial marker 192 comprises or contains amaterial that is MRI visible, such as copper sulphate solution. Thefiducial marker assembly 190 also comprises a circular datum portion 198having a surface protruding through an aperture in the housing thatprovides the externally accessible datum feature 166. A rigid rightangled framework section 200 is also provided to connect the datumfeature 166 to the fiducial marker 192.

As will be explained in more detail below, the fiducial marker assembly190 is retained within the housing part 160 but is free to move relativeto that housing part (i.e. it can be said to be floating orsubstantially unconstrained). This prevents any distortions of thehousing being passed to the fiducial marker assembly and also permitsthe datum feature of the fiducial marker assembly to always adopt thesame position relative to the complementary datum feature of the headclamp even if the housing parts do not adopt repeatable relativepositions. A biasing mechanism 202 is, however, provided to retain thefiducial marker assembly and to bias the datum feature of that assemblyinto engagement with the head clamp during use.

Referring to FIGS. 16 and 17, the biasing mechanism 202 of the firsthousing part 160 described above is shown in cut-away and explodedcut-away views respectively.

The biasing mechanism 202 comprises an x-shaped flexible member 204having four legs with ends that are each secured to the housing part 160by separate screws 206. The centre of the x-shaped flexible member 204engages a spherical protrusion 208 provided at the centre of thecircular datum portion 198 of the fiducial marker assembly 190; thespherical protrusion 208 thus provides a single point of contact withthe substantially flat flexible member 204. The biasing mechanism 202also comprises four flexible loops 210. Each loop 210 is capturedbetween the housing part 160 and one of the legs of the x-shapedflexible member 204 and also engages a protruding feature 212 providedon the circular datum portion 198. The loops are held in slight tensionand, because they are equally spaced around the circumference of thecircular datum portion 198, they maintain the circular datum portion 198in a substantially central position within the aperture of the housing.The flexibility of the loops 210 and x-shaped member 204 does, however,allow movement of the fiducial marker assembly in all 6 degrees offreedom when necessary so as to enable the datum feature 166 to adoptthe necessary position relative to a complementary datum featureprovided on a head clamp. It should also be noted that the biasingmechanism 202 provides the only mechanical connection between thehousing and the fiducial marker assembly and consequently anydistortions of the housing will not be transmitted to that assembly.

The head clamp and imaging apparatus described above are designed to beused together, but it is important to note that each could be usedseparately for different purposes. For example, the head clamp could beused purely for clamping a head during surgical or other procedures.Similarly, the imaging apparatus could be arranged to engage and imageother body parts to which other fixtures are attached.

It should be remembered that the head clamp described above withreference to FIGS. 1 to 11 is also designed to be used in surgicalprocedures. In particular, once an MRI image of the head has beenobtained, the head clamp may be indexed to at least one further positionthat is suitable for conducting a surgical procedure. The, or each,datum feature provided on the head clamp may thus be used to locateother apparatus in a known position relative to the head clamp. Forexample, a datum feature could be used to accurately position, relativeto the head clamp, retro-reflective surgical navigation instruments,surgical robots such as the Renishaw-Mayfield neuromate (RegisteredTrade mark) robot, and targeted radiotherapy devices such as the Leksellgamma-knife (Registered Trade mark) apparatus. Position informationacquired from the MRI images can then be tied back to the datum featureon the head clamp and can therefore be used to precisely target regionsor points in the brain.

Although the mechanical strength of the c-shaped member 4 of the abovedescribed head clamp is designed to be more than adequate for allexpected mechanical loads to which it will be subjected in use, it isnoted that the head clamp may, on very rare occasions, be subjected tolarge mechanical impulses that could exceed safe design limits. Forexample, a surgeon may have no choice other than subjecting the headclamp to large forces to perform an emergency procedure (e.g.resuscitation etc). In the case of a c-shaped member formed from a rigidmaterial, such as a glass filled polymer, any failure may becatastrophic; this can obviously have severe consequences if aneurosurgical procedure is in progress.

Referring to FIG. 18, a supplementary support or backup clamp 230 (e.g.formed from metal) is shown that can be attached to the head clamp 2during surgery. The backup clamp 230 may, for example, attach to theapertures 38 (not visible in FIG. 18 but shown in FIG. 1) formed in thec-shaped member 4 of the head clamp 2. The supplementary clamp 230 isarranged so that access to the datum feature of the head clamp 2 ismaintained. In normal use, the supplementary clamp 230 takes none of themechanical load of the head clamp 2 but it does, however, act as abackup device that takes the full mechanical load of the head clamp 2 ifthat head clamp 2 was to fail. The supplementary clamp 230, which is byno means essential, thus mitigates the unwanted consequences that mightoccur with a catastrophic failure of the head clamp 2.

Referring to FIG. 19, an exploded view is shown of an indexing mechanism328 suitable for inclusion in a head clamp as described above. Theindexing mechanism 328 may, for example, be provided instead of theindexing mechanism 28 described in detail with reference to FIG. 5.

The indexing mechanism 328 comprises a so-called Hirth coupling in whicha series of concentric features 330 are provided around the aperture atthe second end 308 of the c-shaped member 304. An indexable part 300 isalso provided that has a face plate 302 comprising a plurality ofconcentric features 306 that complement the concentric features 330 ofthe c-shaped member 304. In particular, the complementary features 330and 306 are configured such that, when biased into engagement, theindexable part 300 can adopt (i.e. can be indexed into) any one ofmultiple different orientations relative to the c-shaped member 304.

An indexing adjuster 310 and release lever 340 are also provided. Theindexing adjuster 310 is insertable into the aperture at the second end308 of the c-shaped member 304 and includes flexural elements to providea preload bias (i.e. to bias the complementary features 330 and 306 intoengagement even when the head clamp is not loaded) and stops to preventthe part being overloaded. The associated release lever 340 comprises acam 344 and an axle 342. The axle 342 can be attached (clipped into)corresponding slots 346 provided at the second end 308 of the c-shapedmember 304.

Rotation of the release lever 340, when the axle 342 is clipped into theslots 346, allows the cam 344 to engage and disengage the back surfaceof the indexing adjuster 310. Rotation of the release lever 340 can thusbe used to force the cam 344 against the back surface of the indexingadjuster 310 to separate the complementary features 330 and 306. Oncethese features are separated, the indexable part 300 can be indexed intoa different orientation and locked in that orientation by rotating therelease lever 340 to reduce the force applied by the cam 344. In otherwords, the indexing adjuster 310 and release lever 340 allow a user toaxially separate the complementary features 330 and 306 to enable theindexable part 300 to be rotated into a different orientation relativeto the second end 308 of the c-shaped member 304.

The indexable part 300 also comprises a yoke 384 to which a v-shaped pincarrying member (not shown in FIG. 19 but included in FIGS. 20 and 21below) can be pivotally mounted in a similar manner to that describedabove with reference to FIG. 5.

Referring next to FIG. 20, the indexing mechanism 328 of FIG. 19 isshown when assembled. FIG. 20 shows the release lever 340 in thevertical or unlocked orientation in which the cam 344 and indexingadjuster 310 (not visible in FIG. 20) have forced the indexable part 300out of engagement with the features 330 provided at the second end 308of the c-shaped member 304. In this unlocked configuration, the c-shapedmember 304 may be indexed into the required position.

Referring to FIG. 21, the indexing mechanism 328 of FIG. 19 is againshown when assembled but with the release lever 340 in the horizontal orlocked orientation in which its cam 344 (not visible in FIG. 21) doesnot apply a disengaging force via the indexing adjuster 310 (also notvisible in FIG. 21). The features of the indexable part 300 thus engagethe features 330 provided at the second end 308 of the c-shaped member304. In this locked configuration, the c-shaped member 304 is securelyheld in the required indexed position.

A Hirth coupling of this type has the advantage of providing accuratemetrology whilst allowing multiple, repeatable, angular orientations tobe selected. In particular, the arrangement does not necessarily requirea separate locking piece because of the angle-defining features 330 and306 provided at the second end 308 of the c-shaped member 304 and theface plate 302 respectively. These angle-defining features may, forexample, be formed (e.g. ground) in the frame and/or face plate or maybe formed in a component that is subsequently over-moulded into the endof the frame or into the face plate. The Hirth coupling offers lowlevels of backlash and self-centres. A further benefit of the Hirthcoupling is that it is self-locking and resistant to accidental releasewhen under load; the dimensions of the interlocking elements are alsoselected such that the mechanism can only engage in the allowedorientations (e.g. 0° and)±90°. Preferably, all of the components of theindexing mechanism 328 are MRI benign.

A head clamp of the type described above comprises a c-shaped memberhaving an indexing mechanism at one end and a force applicator mechanismat the other end. Each of these mechanisms preferably carry one or morepins for engaging the skull of a subject. Although these pins may becoated with a soft material, they can still present a sharps risk to thepatient when the head clamp is being attached or removed.

The indexing mechanism of the head clamp is preferably lockable invarious indexed positions when not loaded (i.e. when not attached to theskull of a subject). The various skull attachment pins carried by theindexing mechanism are thus prevented from rotating about the clampingaxis when the head clamp is being attached or removed. In contrast, thepins carried by the force applicator mechanism are typically free torotate about the clamping axis until they engage the skull. This freerotation can, in some instances, pose a sharps risk to the patient whomay have their forehead scored from a rotating yoke and associated skullmounting pins during fitting or removal of the head clamp.

Referring to FIGS. 22 to 24, a yoke hold or anti-rotation device 400 isillustrated for reducing the risk of skin injury resulting from the freerotation of skull attachment pins that may be permitted by the forceapplicator mechanism.

FIG. 22 shows the anti-rotation device 400 in an open configuration. Theanti-rotation device 400 comprises a first portion 402 linked to asecond portion 404 by a hinge 406. The first and second portions 402 and404 have a generally half cylindrical shape. The first portion 402comprises a cradle grab 408, a slot 410, a clasp 412 and acircumferential boss 417. The second portion 404 comprises acircumferential boss 414, a protrusion 415 and a pair of ledges 416(only partially visible in FIG. 20) that can mate with the clasp 412 ofthe first portion. The anti-rotation device 400 may be formed as asingle piece from, for example, a suitable polymer. The anti-rotationdevice 400 may be a single use item or a multi-use item that is suitablefor sterilisation.

As shown in FIGS. 23 and 24, the anti-rotation device 400 shown in FIG.22 may be attached to the force applicator mechanism 16 described abovewith reference to FIGS. 1 to 4.

FIG. 23 shows the anti-rotation device 400 in its open configurationwith the first portion 402 placed into contact with the force applicatormechanism 16.

FIG. 24 shows the anti-rotation device 400 after it has been closedaround the force applicator mechanism 16. The pair of ledges 416 andclasp 412 cooperate to provide a snap fit lock that retains theanti-rotation device 400 in position. The slot 410 receives the c-shapedmember 4 and prevents rotation of the anti-rotation device 400 relativeto that c-shaped member 4. The cradle grab 408 and protrusion 415 of theanti-rotation device 400 engage the yoke 52 of the force applicatormechanism 16 and prevent it from rotating. In this manner, the yoke 52is locked in a single orientation relative to the c-shaped member 4.

It should be noted that the anti-rotation device 400 can travel back andforth along the clamping axis with the yoke 52. In other words, theanti-rotation device 400 does not interfere with the normal operation ofthe force applicator mechanism 16 (i.e. which drives the yoke 52 backand forth). The anti-rotation device 400 can thus be used when attachingand/or detaching the head clamp to a subject.

Removal of the anti-rotation device 400 (e.g. by unclipping it) when thehead clamp is secured to a subject permits the c-shaped member to beindexed into the required orientation(s). In this manner, theanti-rotation device 400 reduces the sharps risk associated with usingthe head clamp without an adverse effect on its function.

It should be noted that the anti-rotation device 400 is by no meansessential for operation of the head clamp. It should also be noted thatalternative means for preventing rotation of the yoke carrying the skullattachment pins may be provided. For example, the force applicatormechanism 16 may comprise an integral anti-rotation component. Althoughthe anti-rotation device 400 is described for use with the forceapplicator mechanism 16, the skilled person would also appreciate asimilar device may be used to prevent any unwanted rotation of otherforce applicator mechanisms and/or the indexing mechanism. For example,a similar anti-rotation device may be used for alternative indexingmechanisms that have a freely rotatable yoke prior to engagement of thehead clamp with the skull.

It should again be noted that the above embodiments are merely examplesof the present invention. The skilled person would be aware of the manyvariations and alternative embodiments that would be possible.

1. A method of performing neurosurgery, comprising the steps of; (i)attaching a head clamp comprising a moveable member to the head of asubject, (ii) moving the member into a first position and acquiring atleast one image of the head, (iii) moving the member into a secondposition and performing a neurosurgical procedure, wherein the relativedifference in position between the first position and the secondposition of the member is known.
 2. A method according to claim 1,wherein the head clamp comprises an indexable member and steps (ii) and(iii) comprise indexing the member into the first position and secondposition respectively.
 3. A method according to claim 1, wherein atleast one datum feature is provided on the head clamp that allows imagesacquired during step (ii) to be related to neurosurgical proceduresconducted during step (iii).
 4. A method according to claim 1, whereinstep (i) comprises attaching the head clamp to the head with apredetermined engagement force such that deformations of the member areknown.
 5. A method according to claim 1, wherein step (i) comprisesapplying a clamping force substantially along a clamping axis, whereinthe clamping axis is aligned, at least approximately, with thecantho-meatal plane of the subject's head.
 6. A method according toclaim 1, wherein the member comprises a c-shaped member, wherein thefirst position of step (ii) is a position in which the c-shaped membersubstantially extends around the top of the head thereby allowingimaging of the head and the second position of step (iii) is a positionin which the c-shaped member substantially extends around a side of thehead thereby providing access the head to allow the neurosurgicalprocedure to be performed.